this is a company... we rent grills, ovens and many other things ....and before we send them out for rental we do an in house test ..... I know many other rental companies do the same but I want to be proactive. we can not build an outside test area as space is limited

You will find the answer to your question in Chapter 6 of the 2017 NFPA 58 for the system installation requirements for L-P systems.

You can do it inside your building if the applications comply with 6.2.2. Otherwise, the L-P containers must be kept outside the building and piped to the inside for your testing of the grill in accordance with NFPA 58.

Maybe look to see how the grills you rent out are listed by the manufacturer? Most LP grills are made for outside use only and listed as such, so doing anything with them inside would be a violation of the manufacturers listing and instructions. Of course if their restaurant grills made for inside use, maybe pipe it up as they'd be in a code compliant restaurant allowing for different models to be connected and "tested"? Similarly though, you made need to have a hood system if that is what is required in a commercial kitchen for the given units?